At every crucial moment in recent months, Putin saw the West losing its nerve.

I was in Kiev the night it began, late on the last Thursday in February. Vague and strange reports came in from Ukraine's Crimea. Well-armed men, wearing fatigues but no identifying insignia, were taking over government buildings and setting up road blocks. Phone lines and air links were cut.

The confusion lasted a few hours, followed by sobering clarity. Vladimir Putin was gunning for Ukraine. Barely a week before, this Slavic...